Electrographic printers that use a rotating magnetic brush to apply a dry, particulate developer to a photoconductor member are known in the art. In such electrographic printers, the magnetic brush includes a rotatable magnetic core surrounded by a rotatable, cylindrical toning shell. The toning shell may be eccentrically mounted with respect to the axis of rotation of the magnetic core. The eccentric mounting of the toning shell defines an area of relatively strong magnetic flux where the shell comes closest to the magnetic core, and an area of relatively weak magnetic flux where the shell is farthest away from the core. The magnetic brush is mounted over a developer sump that holds a reservoir of dry, two-component developer including a mixture of ferromagnetic transport particles and toner particles capable of holding an electrostatic charge. A rotatable conveyor roller is disposed between the reservoir of developer in the sump and the toning shell. In operation, the rotatable conveyor roller attracts and transports developer from the sump to a region of the toning shell where the magnetic flux is relatively weak. The rotating toning shell in turn transports the developer toward the photoconductor member. At the line of closest approach between the toning shell and the photoconductor member, the particulate toner component of the developer is transferred to the photoconductor member as a result of electrostatic attraction between the toner particles and the electrostatic field on the photoconductor member and consequently develops a latent electrostatic image on the photoconductor member. The developed image is ultimately transferred to a substrate, such as a sheet of paper, where the toner image is fused into a permanent image via a fuser station. The combination of the magnetic brush, developer sump and conveyor roller is referred to as a developer station in this application.
In order to print images as quickly as possible, it is necessary for the conveyor roller to rapidly deliver developer to the toning shell. However, it is also necessary for the toning shell to deliver a constant, uniform flow of developer material across its width to the photoconductor member by minimizing variations in developer delivery from the sump. Usually, a “metered flow” is obtained by utilizing a metering skive spaced uniformly from the toning shell parallel to the axis of rotation of the toning shell. Such a metered flow of developer on the toning shell insures that a uniform layer or “nap” of developer material of the proper thickness is delivered to the nip with the photoconductor member. If the nap is non-uniform, the resulting image may have streaks or other undesirable artifacts that degrade the quality of the image. If the developer nap is too thick, developer material can jam the nip and be expelled from the developer roller resulting in contamination of other areas of the electrophotographic reproduction apparatus. Finally, if the nap is uniform but too thin, there might not be enough toner present to enable a high quality image.
As previously indicated, past attempts at providing a metered flow of developer material have included the use of a metering skive across the toning shell downstream from the line of developer delivery between the conveyor roller and the toning shell. The skive gap and its relationship to the toning shell must be tightly controlled to achieve both a uniform thickness of the developer nap along the axis of rotation of the developer roller, and a desired thickness that is neither too thick or too thin. Even very small errors in the metering skive gap can result in an unacceptably large error in either the nap uniformity or nap thickness of the developer. Accordingly, when a metering skive is used, it is positioned at the point of the lowest magnetic field strength from the developer roller's magnetic core. It has been found that such positioning significantly decreases the sensitivity of developer nap height to the metering skive gap by a factor of two to four times. This makes the metering skive gap easier to setup in manufacturing and the resulting developer nap thickness less sensitive to differences in that skive gap along the length of the developer roller.
However, the applicants have observed multiple disadvantages associated with such metering skives. First, such metering skives limit the printing speed that can be achieved by the photoconductor member, as they necessarily impose a limit on the amount of developer that the toning shell can deliver to the photoconductor member. Second, the positioning of such skives at the preferred point of the lowest magnetic field strength from the developer roller's magnetic core requires the developer to be applied a relatively large angular distance of nearly 180° away from the line of closest approach between the toning shell and the photoconductor element. Such a large angular distance results in a relatively long residence time for the developer on the toning shell. The applicants have observed that such a relatively long residence time in combination with the rapid rotation of the magnetic core of the brush to achieve high printing speeds can disadvantageously age the developer, rendering it less effective in developing the electrostatic latent image on the photoconductor member. Third, the metered flow obtained with the metering skive is sensitive to the uniformity of the material delivered by the conveyor roller.